Cathode active material, method of preparing the cathode active material, and all-solid-state battery including the same

ABSTRACT

A cathode active material including a first composite oxide represented by Formula (1): 
         x V 2 O 5 .Li 3 PO 4   (1)
         wherein, in Formula 1, x satisfies 2&lt;x≦10.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to and the benefit of Korean PatentApplication No. 10-2016-0122384, filed on Sep. 23, 2016, in the KoreanIntellectual Property Office, and all the benefits accruing therefromunder 35 U.S.C. § 119, the content of which is incorporated herein inits entirety by reference.

BACKGROUND 1. Field

The present disclosure relates to a cathode active material, a method ofpreparing the cathode active material, and an all-solid-state batteryincluding the same.

2. Description of the Related Art

A lithium ion secondary battery has a high charge/discharge capacity, ahigh operation potential, and excellent charge/discharge cyclecharacteristics. Thus the demand for their use in portable informationterminals, portable electronic devices, and small-sized electric powerstorage devices, as well as in motor cycles, electric vehicles, andhybrid electric vehicles having a motor as a power source, hasincreased. The battery uses an electrolyte solution that includes alithium salt in an organic solvent.

However, the non-aqueous electrolyte solution is both flammable andcapable of leakage. Therefore, in recent years, developing anall-solid-state lithium battery having a solid electrolyte formed of aninorganic material has become a priority. A solid electrolyte isattractive because it may be less flammable and less susceptible toleakage.

A sulfide or an oxide may be used as a solid electrolyte, and sulfidesolid electrolytes provide lithium ion conductivity. When cathode activematerials such nickel cobalt aluminum acid(LiNi_(0.8)Co_(0.15)Al_(0.05)O₂, also referred to as “NCA”) or lithiumcobalt oxide (LiCoO₂, also referred to as “LCO”) are used with thesesolid electrolytes, reactions can occur between the cathode activematerial and the solid electrolyte at their interface duringcharge/discharge cycles. An interface resistance is thus produced andlithium ion conductivity deteriorates. Additionally, the chargingvoltage using these materials is limited to 4.0 V or less. Other cathodeactive materials for use in an all-solid-state battery include vanadiumpentoxide (V₂O₅) and phosphorus pentoxide (P₂O₅). However, capacitydeterioration may still occur with these materials during repeatedcharge/discharge processes.

Furthermore, the charging voltage of a lithium ion secondary batteryhaving an electrolyte solution may be 4.2 V or higher, so the energydensity of an all-solid-state lithium battery needs to be improved.

A cathode active material having a capacity, charging voltage, andcapacity retention that are comparable to an aqueous-based lithium ionsecondary battery is desirable. Therefore, there remains a need for acathode active material that is capable of improving discharge capacityand cycle characteristics for use in an all-solid-state lithium batterythat has a solid electrolyte.

SUMMARY

Provided is an all-solid-state secondary battery having an improveddischarge capacity and cycle characteristics, the all-solid-statesecondary battery including a cathode active material, and a cathodeincluding the cathode active material.

Additional aspects will be set forth in part in the description whichfollows and, in part, will be apparent from the description, or may belearned by practice of the presented embodiments.

According to an embodiment, a cathode active material includes a firstcomposite oxide represented by Formula (1):

xV₂O₅.Li₃PO₄  (1)

wherein, in Formula (1), x satisfies 2<x≦10.

According to another embodiment, a method of preparing a cathode activematerial includes contacting vanadium oxide and lithium phosphoric acidto prepare a mixture; and milling the mixture to obtain a compositeoxide to prepare the cathode active material.

According to another embodiment, an all-solid-state secondary batteryincludes a cathode including the cathode active material; an anode; anda solid electrolyte layer disposed between the cathode and the anode.

BRIEF DESCRIPTION OF THE DRAWINGS

These and/or other aspects will become apparent and more readilyappreciated from the following description of the embodiments, taken inconjunction with the accompanying drawings, in which:

FIG. 1 is a cross-sectional view illustrating an exemplary embodiment ofa structure of an all-solid-state secondary battery;

FIG. 2 is a graph of voltage (volts, V, versus Li/Li⁺) versus capacity(milliampere hours per gram, mAh/g) and shows charge/discharge traces ofan all-solid-state secondary battery including a cathode using3V₂O₅—Li₃PO₄ as a cathode active material according to an embodiment andan all-solid-state secondary battery including a cathode usingnickel-cobalt-manganese(NCM) as a cathode active material;

FIG. 3 is a graph of voltage (V versus Li/Li⁺) versus capacity (mAh/g)and shows charge/discharge traces of all-solid-state secondary batteriesincluding a cathode using 3V₂O₅—Li₃PO₄ as a cathode active materialaccording to an embodiment, a cathode using 3V₂O₅—Li₃PO₄ mixed withLi₂MoO₄ as a cathode active material, and a cathode using a cathodeactive material of 3V₂O₅—Li₃PO₄ having a surface thereof coated withLi₂ZrO₃;

FIG. 4 is a graph of voltage (V versus Li/Li⁺) versus capacity (mAh/g)and shows charge/discharge traces of Example 1 and Comparative Example1;

FIG. 5 is a graph of discharge capacity (mAh/g) versus x in the formulaxV₂O₅.(1−x)Li₃PO₄ and shows a trace that illustrates the change of asecondary discharge capacity according to a ratio of V₂O₅ and Li₃PO₄ forExamples 1 to 4 and Comparative Examples 1 and 2; and

FIGS. 6A and 6B are SEM images of 4V₂O₅—Li₃PO₄ in the cathode activematerial according to an embodiment.

DETAILED DESCRIPTION

Reference will now be made in detail to embodiments of a cathode activematerial, a method of preparing a cathode active material, and anall-solid-state battery including a cathode including the cathode activematerial, embodiments of which are illustrated in the accompanyingdrawings, wherein like reference numerals refer to like elementsthroughout. In this regard, the disclosed embodiments may have differentforms and should not be construed as being limited to the descriptionsset forth herein. Accordingly, the embodiments are described below, byreferring to the figures, to explain aspects.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting. As used herein, thesingular forms “a,” “an,” and “the” are intended to include the pluralforms, including “at least one,” unless the content clearly indicatesotherwise. “Or” means “and/or.” As used herein, the term “and/or”includes any and all combinations of one or more of the associatedlisted items. Expressions such as “at least one of,” when preceding alist of elements, modify the entire list of elements and do not modifythe individual elements of the list.

In addition, unless explicitly described to the contrary, the word“comprise” and variations such as “comprises” or “comprising”, will beunderstood to imply the inclusion of stated elements but not theexclusion of any other elements. It will be further understood that theterms “comprises” and/or “comprising,” or “includes” and/or “including”when used in this specification, specify the presence of statedfeatures, regions, integers, steps, operations, elements, and/orcomponents, but do not preclude the presence or addition of one or moreother features, regions, integers, steps, operations, elements,components, and/or groups thereof.

It will be understood that when an element is referred to as being “on”another element, it can be directly on the other element or interveningelements may be present therebetween. In contrast, when an element isreferred to as being “directly on” another element, there are nointervening elements present.

It will be understood that, although the terms “first,” “second,”“third,” etc. may be used herein to describe various elements,components, regions, layers, and/or sections, these elements,components, regions, layers, and/or sections should not be limited bythese terms. These terms are only used to distinguish one element,component, region, layer, or section from another element, component,region, layer or section. Thus, “a first element,” “component,”“region,” “layer,” or “section” discussed below could be termed a secondelement, component, region, layer, or section without departing from theteachings herein.

“About” or “approximately” as used herein is inclusive of the statedvalue and means within an acceptable range of deviation for theparticular value as determined by one of ordinary skill in the art,considering the measurement in question and the error associated withmeasurement of the particular quantity (i.e., the limitations of themeasurement system). For example, “about” can mean within one or morestandard deviations, or within ±30%, 20%, 10%, or 5% of the statedvalue.

Exemplary embodiments are described herein with reference to crosssection illustrations that are schematic illustrations of idealizedembodiments. As such, variations from the shapes of the illustrations asa result, for example, of manufacturing techniques and/or tolerances,are to be expected. Thus, embodiments described herein should not beconstrued as limited to the particular shapes of regions as illustratedherein but are to include deviations in shapes that result, for example,from manufacturing. For example, a region illustrated or described asflat may, typically, have rough and/or nonlinear features. Moreover,sharp angles that are illustrated may be rounded. Thus, the regionsillustrated in the figures are schematic in nature and their shapes arenot intended to illustrate the precise shape of a region and are notintended to limit the scope of the present claims.

As used herein, “is a Group 3 to Group 15 element” means a Group 3element, a Group 4 element, a Group 5 element, a Group 6 element, aGroup 7 element, a Group 8 element, a Group 9 element, a Group 10element, a Group 11 element, a Group 12 element, a Group 13 element, aGroup 14 element, a Group 15 element, or a combination including atleast one of the foregoing.

“Group” means a group of the periodic table of the elements according tothe International Union of Pure and Applied Chemistry (“IUPAC”) 1-18Group classification system.

Cathode Active Material

A cathode active material according to an embodiment may include a firstcomposite oxide represented by Formula (1):

xV₂O₅.Li₃PO₄  (1)

In Formula (1), x satisfies 2<x≦10.

In Formula (1), x satisfies 2<x<10, preferably 3≦x<10.

Vanadium pentoxide (V₂O₅) is known as having a theoretical capacity thatis about twice the capacity of each of lithium cobalt oxide (LiCoO₂,also referred to as “LCO”) and nickel cobalt aluminum acid(LiNi_(0.8)Co_(0.15)Al_(0.05)O₂, also referred to as “NCA”) but has aproblem of significant capacity deterioration upon charging/dischargingcycles. In this regard, the present inventors have examined a method forreducing the problem of capacity deterioration while maintaining a highcapacity of vanadium pentoxide, and as a result, have developed acomposite oxide in the form of solid vanadium pentoxide particles andsolid electrolyte particles.

Without being bound by theory, the capacity deterioration of the cathodeactive material including the composite oxide may be significantlydecreased by using lithium phosphate (Li₃PO₄) as the solid electrolyteparticles included in the composite oxide and controlling a molar ratioof the vanadium pentoxide particles to the lithium phosphate particles.

Additionally, and without being bound by theory, when a molar ratio ofthe vanadium pentoxide particles to the lithium phosphate particles in amixture thereof is 3:1, the capacity deterioration significantlydecreases as compared to the case when the molar ratio is 2:1. In thismanner, when a proportion of vanadium pentoxide increases, it is deemedthat the capacity deterioration does not occur because lithium phosphatehas lithium ion conductivity and is chemically stable. When a molarratio of vanadium pentoxide particles to lithium phosphate particleswhen mixed is 3:1, surfaces of vanadium pentoxide particles are thinlyand evenly coated with lithium phosphate. Therefore, it is deemed that areaction between the vanadium pentoxide particles and the solidelectrolyte particles, for example lithium phosphate particles, may besuppressed, and thus capacity deterioration may be prevented. When amolar ratio of the vanadium pentoxide particles to the lithium phosphateparticles is greater than 10:1, the remaining vanadium pentoxideparticles that are not coated with lithium phosphate react with solidelectrolyte particles, and thus, may result in significant capacitydeterioration. When a molar ratio of the vanadium pentoxide particles tolithium phosphate particles in the composite oxide is controlled to arange of greater than about 2:1 to about 10:1 or less, the capacitydeterioration during charging/discharging may be reduced, for exampleprevented, and thus an all-solid-state secondary battery using a cathodeincluding the composite oxide may have a high capacity and high lifespancharacteristics.

The cathode active material may further include a lithium metal oxidecompound represented by Formula (1A):

Li_(a)M_(b)O_(c)  (1A).

In Formula (1A), M is a Group 3 to Group 15 element or a combinationcomprising at least one of a Group 3 to Group 15 element, and a, b, andc respectively satisfy 1≦a≦2, 0<b≦1, and 1<c≦4.

In an embodiment, the cathode active material may include a secondcomposite oxide that is represented by Formula (2):

y(xV₂O₅.Li₃PO₄).(100−y)Li_(a)M_(b)O_(c)  (2).

In Formula (2), M is a Group 3 to Group 15 element or a combinationcomprising at least one of a Group 3 to Group 15 element, and x, y, a,b, and c respectively satisfy 2<x≦10, 95≦y<100, 1≦a≦2, 0<b≦1, and 1<c≦4.

In Formula (2), M may be B, Zr, Nb, Mo, W, or a combination including atleast one of the foregoing, but embodiments are not limited thereto.

For example, the second composite oxide represented by Formula (2) is acomposite oxide represented by y(xV₂O₅.Li₃PO₄).(100−y)LiNbO₃,y(xV₂O₅.Li₃PO₄).(100−y)Li₂MoO₄, y(xV₂O₅.Li₃PO₄).(100−y)LiBO₂,y(xV₂O₅.Li₃PO₄).(100−y)Li₂WO₄, or y(xV₂O₅.Li₃PO₄).(100−y)Li₂ZrO₃ whereinx and y respectively satisfy 2<x≦10 and 95≦y<100.

For example, the second composite oxide represented by Formula (2) maybe a composite oxide represented by 95(4V₂O₅.Li₃PO₄).5LiNbO₃,95(4V₂O₅.Li₃PO₄).5Li₂MoO₄, 95(4V₂O₅.Li₃PO₄).5LiBO₂,95(4V₂O₅.Li₃PO₄).5Li₂WO₄, 99(4V₂O₅.Li₃PO₄).1Li₂MoO₄, or99(4V₂O₅.Li₃PO₄).1Li₂ZrO₃.

When a portion of the first composite oxide is substituted with thesecond composite oxide, a resistance increase at an interface between asulfide-based solid electrolyte layer and a cathode may be suppressed,and, as a result, ion conductivity may improve, which may result in anincrease in capacity.

A cathode active material according to another embodiment may include athird composite oxide including a core and a coating layer disposed onthe core. The core may include the first composite oxide represented byFormula (1), and the coating layer may include a Group 3 to Group 15element or a combination including at least one of a Group 3 to Group 15element.

The coating element included in the coating layer may be Sc, Y, Ti, Zr,Hf, Rf, V, Nb, Ta, Db, Cr, Mn, W, Sg, Mn, Tc, Re, Fe, Ru, Os, Hs, Co,Rh, Ir, Mt, Ni, Pd, Pt, Ds, Cu, Ag, Au, Rg, Zn, Cd, Hg, Cn, B, Al, Ga,In, Ti, C, Si, Ge, Sn, Pb, N, P, As, Sb, or a combination including atleast one of the foregoing.

For example, the coating element may include B, Zr, Nb, Mo, W, or acombination including at least one of the foregoing, but embodiments arenot limited thereto. For example, the coating element of the coatinglayer may include Zr. In another embodiment, the coating element is notlimited thereto.

The coating layer may include a coating element compound which is anoxide, a hydroxide, an oxyhydroxide, an oxycarbonate, or ahydroxycarbonate of the coating element. For example, the coating layermay include an oxide of a coating element.

The compound included in the coating layer may be amorphous orcrystalline.

In another embodiment, the coating layer may include a compoundrepresented by Formula (1B):

Li_(a)M_(b)O_(c)  (1B)

In Formula (1B), M may be a Group 3 to Group 15 element or a combinationincluding at least one of a Group 3 to Group 15 element, and a, b, and crespectively may satisfy 1≦a≦2, 0<b≦1, and 1<c≦4.

In an embodiment, in Formula (1B), M may include B, Zr, Nb, Mo, or W, ora combination including at least one of the foregoing, but embodimentsare not limited thereto.

The coating layer may include LiNbO₃, Li₂MoO₄, LiBO₂, Li₂WO₄, Li₂ZrO₃,or a combination including at least one of the foregoing, butembodiments are not limited thereto. For example, the coating layer mayinclude Li₂ZrO₃.

An amount of the coating layer may be in a range of about 0.1 weightpercent (wt %) to about 10 wt %, for example, in a range of about 0.5 wt% to about 10 wt %, for example, about 1 wt % to about 10 wt %, forexample, about 0.1 wt % to about 9 wt %, or, for example, about 0.1 wt %to about 5 wt %, based on the total weight of the third composite oxide,but embodiments are not limited thereto, and may include any subrangewithin these ranges.

When the amount of the coating layer is within these ranges, resistanceat an interface between the cathode active material layer and the solidelectrolyte layer may be decreased.

An average particle diameter of the first composite oxide may be in arange of about 0.1 micrometer (μm) to about 10 μm, for example, about0.1 μm to about 5 μm, or, for example, about 1 μm to about 10 μm, butembodiments are not limited thereto, and may include any subrange withinthese ranges.

In an embodiment, a discharge capacity at the 2^(nd) cycle of a lithiumbattery including the first composite oxide is 270 milliampere hours pergram (mAh/g) or greater.

Method of Preparing Cathode Active Material

A method of preparing a cathode active material is not particularlylimited, and, for example, a cathode active material may be prepared asfollows.

According to another embodiment, a method of preparing a cathode activematerial includes contacting a vanadium oxide, for example vanadiumpentoxide (V₂O₅), and lithium phosphoric acid, for example lithiumphosphate (Li₃PO₄), to prepare a mixture; and milling the mixture toobtain a composite oxide to prepare the cathode active material. In anembodiment, the milling is mechanical milling.

In an embodiment, a molar ratio of vanadium pentoxide to lithiumphosphate may be in a range of greater than about 2:1 to about 10:1 orless, for example, greater than 2:1 to less than 10:1, or, for example,about 3:1 or greater to less than about 10:1, but embodiments are notlimited thereto, and may include any molar ratio within these ranges.

Also, the mixture may further include the compound represented byFormula (1A):

Li_(a)M_(b)O_(c),  (1A)

In Formula (1A), M is a Group 3 to Group 15 element or a combinationincluding at least one of a Group 3 to Group 15 element, and a, b, and crespectively satisfy 1≦a≦2, 0<b≦1, and 1<c≦4.

In an embodiment, the preparation method may further include contacting,for example adding to and/or mixing with, a coating solution and thecomposite oxide, wherein the coating solution comprises a compound ofFormula (1B):

Li_(a)M_(b)O_(c)  (1B),

wherein, in Formula (1B), M is a Group 3 to Group 15 element or acombination including at least one of a Group 3 to Group 15 element, anda, b, and c respectively satisfy 1≦a≦2, 0<b≦1, and 1<c≦4, to obtain amixed solution; and drying and heat-treating the mixed solution underoxidative conditions, for example in the presence of oxygen or theambient atmosphere, to obtain a Li_(a)M_(b)O_(c)-coated composite oxide.

The heat-treating may be performed at a temperature in a range of about300° C. to about 500° C., for example, about 300° C. to about 450° C.,or for example, 300° C. to about 400° C. For example, the heat-treatingmay be performed at a temperature of about 350° C. In an embodiment, theheat-treating may be performed for, for example, about 1 hour. In thisregard, a cathode active material having a lithium metal oxide on asurface thereof may be obtained.

All-Solid-State Secondary Battery

According to another embodiment, provided is an all-solid-statesecondary battery including a cathode including the cathode activematerial according to an embodiment; an anode; and a solid electrolytelayer disposed between the cathode and the anode.

Cathode

A cathode may be prepared as follows. In an embodiment, a cathode activematerial, a solid electrolyte, a conducting agent, a binder, and asolvent are combined, for example mixed, to prepare a cathode activematerial composition. In an embodiment, the cathode active materialcomposition may be coated, for example directly coated, and dried on acurrent collector, for example an aluminum current collector, to preparea cathode plate on which a cathode active material layer is formed. Inan embodiment, the cathode active material composition may be formed,for example cast, on a separate support to form a cathode activematerial film, which may then be separated from the support and appliedto, for example laminated on, a current collector, for example analuminum current collector, to prepare a cathode plate on which acathode active material layer is formed.

Examples of the conducting agent may include carbon black, graphiteparticulates, natural graphite, artificial graphite, acetylene black,Ketjen black, and carbon fibers; carbon nanotubes; metal powder, metalfibers, and metal tubes of copper, nickel, aluminum, and silver; and aconductive polymer such as a polyphenylene derivative, but embodimentsare not limited thereto, and any suitable material available as aconducting agent, including those available in the art, may be used.

Examples of the binder may include a vinylidenefluoride/hexafluoropropylene copolymer, polyvinylidene fluoride (PVDF),polyacrylonitrile, polymethylmethacrylate, polytetrafluoroethylene(PTFE), mixtures thereof, and a styrene-butadiene rubber polymer, butembodiments are not limited thereto. Any suitable material available asa binding agent, including those available in the art, may be used.Examples of the solvent may include toluene, xylene, and hexane, butembodiments are not limited thereto. Any suitable material available asa solvent, including those available in the art, may be used.

In an embodiment, a plasticizing agent may be further added to thecathode active material composition to form pores in the electrodeplate.

The amounts of the cathode active material, the conducting agent, thebinder, and the solvent may be in ranges suitable for lithium batteries.In an embodiment, at least one of the conducting agent, the binder, andthe solvent may be omitted according to the use and the structure of thelithium battery.

Anode

An anode may include lithium or a lithium alloy. Examples of the lithiumalloy may include aluminum-lithium, indium-lithium, tin-lithium,lead-lithium, silver-lithium, and copper-lithium.

For example, the anode may include an aluminum-lithium alloy. The anodeincluding the aluminum-lithium alloy may be prepared by putting asubstantially pure aluminum bar into an electrolyte containing lithiumions to charge lithium ions into the aluminum bar, for example in themanner of dispersion.

In an embodiment, a lithium alloy used in the anode may be prepared byusing another suitable technique, including those known in the art.Other suitable anode materials may be used, including those available inthe art.

Solid Electrolyte Layer

A solid electrolyte layer includes a sulfide-based solid electrolyte(“solid electrolyte”). The sulfide-based solid electrolyte may at leastinclude sulfur (S) and lithium (Li), and may further include phosphorus(P), silicon (Si), boron (B), aluminum (Al), germanium (Ge), zinc (Zn),gallium (Ga), indium (In), one or more halogen elements, or acombination including at least one of the foregoing. For example, thesulfide-based solid electrolyte may include S and Li, and may furtherinclude Si, P, B, or a combination including at least one of theforegoing. The solid electrolyte that satisfies these conditions, i.e.,the sulfide-based solid electrolyte, has lithium ion conductivity higherthan that of other inorganic compounds.

An example of the sulfide-based solid electrolyte may include Li₂S—P₂S₅.The solid electrolyte may be obtained by heating Li₂S and P₂S₅ to amelting temperature or greater, mixing the melted Li₂S and P₂S₅ at apredetermined ratio to obtained a melted mixture, maintaining the meltedmixture for a predetermined time, and rapidly cooling the mixture. In anembodiment, a powder of Li₂S and P₂S₅ may be by treating the cooledmixture by using a milling method, for example a mechanical millingmethod. In another embodiment, a suitable sulfide-based solidelectrolyte may be prepared using a suitable method, including thosesolid electrolytes and methods available in the art.

EXAMPLES Example 1 Preparation of Cathode Active Material

Vanadium pentoxide (V₂O₅) and lithium phosphate (Li₃PO₄) aremechanically milled at a molar ratio of V₂O₅:Li₃PO₄=3:1 to obtain acathode active material.

Preparation of Sulfide-Based Solid Electrolyte

Lithium sulfide (Li₂S) and diphosphorus pentasulfide (P₂S₅) aremechanically milled at a molar ratio of Li:P=8:2 to obtain asulfide-based solid electrolyte.

Preparation of Cell for Test

An all-solid-state secondary battery is prepared in an inert gasatmosphere as follows.

The cathode active material, the sulfide-based solid electrolyte, and acarbonaceous material, as a conducting material, were homogenously mixedat a mixing ratio of 44/49/7 (wt %) by using a mortar to obtain acathode mixture. 30 mg of the cathode mixture thus obtained was insertedinto a molding jig and press-molded under a pressure of 2 tons persquare centimeter (ton/cm²) to prepare a cathode mixture in the form ofa pellet.

The pelletized cathode mixture was disposed on a stainless steel currentcollector to prepare a cathode layer.

150 mg of the sulfide-based solid electrolyte powder was inserted into amolding jig and press-molded under a pressure of 2 ton/cm², and thus asolid electrolyte layer was prepared. The cathode layer was insertedinto the same molding jig and press-molded under a pressure of 2ton/cm², and thus the solid electrolyte layer and the cathode layer wereintegrated as one body.

Then, a Li metal foil having a thickness of 100 μm, as an anode mixture,was inserted to the same molding jig so that the solid electrolyte layermay contact the cathode layer and the anode layer, and press-moldedunder a pressure of 3 ton/cm² and thus the solid electrolyte layer, thecathode layer, and the anode layer were integrated into one body. Inthis manner, a test cell was obtained.

Examples 2 to 4

Using a molar ratio of V₂O₅ to Li₃PO₄ of 4:1, 5:1, and 10:1,respectively, the test cells were prepared in the same manner as inExample 1.

Examples 5 to 9

A lithium metal oxide was added to Example 2 (V₂O₅:Li₃PO₄=4:1), and thetest cells were prepared in the same manner as in Example 1.

Example 10

A coating treatment was performed with a lithium metal oxide on Example2 (V₂O₅: Li₃PO₄=4:1) as follows.

Coating treatment process

0.08 g of a 10% lithium methoxide in methanol solution and 0.03 g ofzirconium(IV) propoxide were mixed and dissolved in an isopropanolsolution for 30 minutes. 1 g of the cathode active material prepared inExample 2 was added to the solution. The mixed solution thus obtainedwas heated to a temperature of 40° C. and stirred to evaporate thesolvent until dry. The drying process was performed while sonicating themixed solution. In this regard, a reaction precursor of alithium-zirconium oxide was deposited on a surface of the cathode activematerial. In addition, the lithium-zirconium oxide precursor depositedon the surface of the cathode active material was heat-treated in anoxygen atmosphere at 350° C. for 1 hour. As a result, the coatingcathode active material was coated with 1 mol % of lithium-zirconiumoxide.

Comparative Example 1

A test cell was prepared in the same manner as in Example 1, except thatlithium phosphate was not used.

Comparative Example 2

A test cell was prepared in the same manner as in Example 1, except thata molar ratio of V₂O₅:Li₃PO₄ was 2:1.

Comparative Example 3

A test cell was prepared in the same manner as in Example 1, except amolar ratio of V₂O₅:Li₃PO₄=4:1 was used as in Example 2, and P₂O₅ wasadded thereto.

Cycle Lifespan Test

The test cell thus obtained was used to perform a 0.05 Cconstant-current charge/discharge cycle test at room temperature (25°C.). In particular, the test cell underwent two charge/discharge cycles,each of the cycles including discharging the test cell with a constantcurrent of 0.13 milliamperes (mA) until a lower-limit voltage of 1.5volts (V) was reached and then charging with a constant current of 0.13mA until a voltage of 4.0 V was reached at 25° C. In addition, apercentage of a discharge capacity at the 2^(nd) cycle (“secondarydischarge capacity”) to a discharge capacity (an initial capacity) atthe 1^(st) cycle (“primary discharge capacity”) was defined as adischarge capacity retention. The discharge capacity retention is aparameter that represents cycle characteristics, and as the dischargecapacity retention increases, the cycle characteristics improve.

The results of the cycle lifespan test performed on Examples 1 to 10 andComparative Examples 1 to 3 are shown in Table 1.

TABLE 1 Discharge capacity Primary discharge Secondary dischargeDischarge Capacity Composition (mAh/g) (mAh/g) Retention Example 13V₂O₅·Li₃PO₄ 313 299 95% Example 2 4V₂O₅·Li₃PO₄ 348 297 85% Example 35V₂O₅·Li₃PO₄ 360 321 89% Example 4 10V₂O₅·Li₃PO₄ 372 300 81% Example 595(4V₂O₅·Li₃PO₄)·5LiNbO₃ 329 287 87% Example 6 95(4V₂O₅·Li₃PO₄)·5Li₂MoO₄351 317 90% Example 7 95(4V₂O₅·Li₃PO₄)·5LiBO₂ 319 304 95% Example 895(4V₂O₅·Li₃PO₄)·5Li₂WO₄ 323 283 88% Example 9 99(4V₂O₅·Li₃PO₄)·1Li₂MoO₄327 315 96% Example 10 99(4V₂O₅·Li₃PO₄)·1Li₂ZrO₃ 330 315 96% ComparativeV₂O₅ 384 255 66% Example 1 Comparative 2V₂O₅·Li₃PO₄ 281 229 82% Example2 Comparative 95(4V₂O₅·Li₃PO₄)·5P₂O₅ 218 212 97% Example 3

Referring to FIG. 2, the composite oxide having the composition ofExample 1 has a high capacity compared to that of NCM, which has beenused as a cathode active material in an all-solid-state secondarybattery.

In addition, referring to FIG. 3, a capacity of the test cell havingLi₂MoO₄ added to the first composite oxide as an additive was increasedas compared with a capacity of the composite oxide having thecomposition of Example 1. Moreover, when the composite oxide of Example1 was coated with Li₂ZrO₃, the capacity further increased as comparedwith that of the test cell when the additive was used.

Referring to Examples 1 to 4, Comparative Examples 1 and 2, FIGS. 4 and5, and Table 1, the discharge capacity retentions were significantlyimproved in cases when a composite oxide of vanadium pentoxide andlithium phosphate were used as compared with those of cases in whichvanadium pentoxide was used alone. Particularly, when a molar ratio ofvanadium pentoxide to lithium phosphate was greater than 2:1, thedischarge capacity retentions significantly improved.

In addition, according to Examples 5 to 9, some of the composite oxidesof vanadium pentoxide and lithium phosphate were substituted withlithium metal oxides, and thus the discharge capacity retentions werefurther improved as compared with those of the composite oxides.

Further, according to Example 10 and Comparative Example 3, when azirconium coating layer is formed on the composite oxide of vanadiumpentoxide and lithium phosphate, high capacity characteristics of thecomposite oxide were maintained, and a discharge capacity retentionequivalent to that when diphosphorus pentoxide was added was achieved.

In conclusion, when an all-solid-state secondary battery uses a cathodeincluding the cathode active material according to an embodiment and asulfide-based solid electrolyte, the all-solid-state secondary batterymay have high capacity and high lifespan characteristics.

As described above, according to an embodiment, discharge capacity andcycle characteristics of an all-solid-state secondary battery using asolid electrolyte layer including a sulfide-based solid electrolyte mayimprove due to a cathode active material including a first compositeoxide of an embodiment.

It should be understood that an embodiment described herein should beconsidered in a descriptive sense only and not for purposes oflimitation. Descriptions of features or aspects within each embodimentshould typically be considered as available for other similar featuresor aspects in another embodiment.

While one or more embodiments have been described with reference to thefigures, it will be understood by those of ordinary skill in the artthat various changes in form and details may be made therein withoutdeparting from the spirit and scope as defined by the following claims.

What is claimed is:
 1. A cathode active material comprising: a firstcomposite oxide represented by Formula (1):xV₂O₅.Li₃PO₄  (1) wherein, in Formula (1), x satisfies 2<x≦10.
 2. Thecathode active material of claim 1, further comprising a compoundrepresented by Formula (1A):Li_(a)M_(b)O_(c)  (1A) wherein, in Formula (1A), M is a Group 3 to Group15 element or a combination comprising at least one of a Group 3 toGroup 15 element, and a, b, and c respectively satisfy 1≦a≦2, 0<b≦1, and1<c≦4.
 3. The cathode active material of claim 1, wherein the cathodeactive material comprises a second composite oxide represented byFormula (2):y(xV₂O₅.Li₃PO₄).(100−y)Li_(a)M_(b)O_(c)  (2) wherein, in Formula (2), Mis a Group 3 to Group 15 element or a combination comprising at leastone of a Group 3 to Group 15 element, and x, y, a, b, and c respectivelysatisfy 2<x≦10, 95≦y<100, 1≦a≦2, 0<b≦1, and 1<c≦4.
 4. The cathode activematerial of claim 3, wherein in Formula (2), M is B, Zr, Nb, Mo, W, or acombination comprising at least one of the foregoing.
 5. The cathodeactive material of claim 3, wherein the second composite oxiderepresented by Formula (2) is y(xV₂O₅.Li₃PO₄).(100−y)LiNbO₃,y(xV₂O₅.Li₃PO₄).(100−y)Li₂MoO₄, y(xV₂O₅.Li₃PO₄).(100−y)LiBO₂,y(xV₂O₅.Li₃PO₄).(100−y)Li₂WO₄, or y(xV₂O₅.Li₃PO₄).(100−y)Li₂ZrO₃ whereinx and y respectively satisfy 2<x≦10 and 95≦y<100.
 6. The cathode activematerial of claim 3, wherein the second composite oxide represented byFormula (2) is 95(4V₂O₅.Li₃PO₄).5LiNbO₃, 95(4V₂O₅.Li₃PO₄).5Li₂MoO₄,95(4V₂O₅.Li₃PO₄).5LiBO₂, 95(4V₂O₅.Li₃PO₄).5Li₂WO₄,99(4V₂O₅.Li₃PO₄).1Li₂MoO₄, or 99(4V₂O₅.Li₃PO₄).1Li₂ZrO₃.
 7. The cathodeactive material of claim 1, further comprising a third composite oxide,the third composite oxide comprising: a core; and a coating layerdisposed on the core, wherein the core comprises the first compositeoxide represented by Formula (1), and wherein the coating layercomprises a Group 3 to Group 15 element.
 8. The cathode active materialof claim 7, wherein the coating layer comprises B, Zr, Nb, Mo, W, or acombination comprising at least one of the foregoing.
 9. The cathodeactive material of claim 7, wherein an amount of the coating layer is ina range of about 0.1 weight percent to about 10 weight percent, based ona total weight of the third composite oxide.
 10. The cathode activematerial of claim 7, wherein the coating layer comprises a compoundrepresented by Formula (1B):Li_(a)M_(b)O_(c)  (1B) wherein, in Formula (1B), M is a Group 3 to Group15 element or a combination comprising at least one of a Group 3 toGroup 15 element, and a, b, and c respectively satisfy 1≦a≦2, 0<b≦1, and1<c≦4.
 11. The cathode active material of claim 10, wherein M comprisesB, Zr, Nb, Mo, W, or a combination comprising at least one of theforegoing.
 12. The cathode active material of claim 10, wherein thecoating layer comprises LiNbO₃, Li₂MoO₄, LiBO₂, Li₂WO₄, Li₂ZrO₃, or acombination comprising at least one of the foregoing.
 13. The cathodeactive material of claim 1, wherein an average particle diameter of thefirst composite oxide is in a range of about 0.1 micrometer to about 10micrometers.
 14. The cathode active material of claim 1, wherein adischarge capacity at a second cycle of a lithium battery comprising thefirst composite oxide is 270 milliamperes per gram or greater.
 15. Amethod of preparing a cathode active material, the method comprising:contacting vanadium oxide and lithium phosphoric acid to prepare amixture; and milling the mixture to obtain a composite oxide to preparethe cathode active material.
 16. The method of claim 15, wherein thevanadium oxide and lithium phosphoric acid are contacted at a molarratio in a range of greater than about 2:1 to about 10:1 or less. 17.The method of claim 15, wherein the mixture further comprises a compoundrepresented by Formula (1A):Li_(a)M_(b)O_(c)  (1A) wherein, in Formula (1A), M is a Group 3 to Group15 element or a combination comprising at least one of a Group 3 toGroup 15 element, and a, b, and c respectively satisfy 1≦a≦2, 0<b≦1, and1<c≦4.
 18. The method of claim 15, further comprising: contacting acoating solution and the composite oxide, wherein the coating solutioncomprises a compound of Formula (1B):Li_(a)M_(b)O_(c)  (1B), wherein, in Formula (1B), M is a Group 3 toGroup 15 element or a combination comprising at least one of a Group 3to Group 15 element, and a, b, and c respectively satisfy 1≦a≦2, 0<b≦1,and 1<c≦4, to obtain a mixed solution; and drying and heat-treating themixed solution under oxidative conditions to obtain aLi_(a)M_(b)O_(c)-coated composite oxide.
 19. An all-solid-statesecondary battery comprising: a cathode comprising the cathode activematerial of claim 1; an anode; and a solid electrolyte layer disposedbetween the cathode and the anode.
 20. The all-solid-state secondarybattery of claim 19, wherein the solid electrolyte layer comprises asolid electrolyte, wherein the solid electrolyte comprises S, Li, andSi, P, B, or a combination comprising at least one of the foregoing.